The reason I have trouble trusting closed-source crypto is that users don't know when it fails, so they can't judge good from bad. Companies that write closed source crypto exist to make money, and there's good money in backdooring your system for the government. Users are happy (since they don't know), the government is happy (since they get the information), and the company is happy (since it's making money on both sides of the deal). In short: the incentives simply don't align in favor of the user.
Weren't there allegations precisely to this effect that RSA took millions from the government to make Dual_EC_DRBG the default in BSafe? I don't know if it's true, but the fact that it's plausible is a problem for me.